Expo is actually when a person makes the controls for elevator, ailerons, etc either more or less sensitive in the center of the stick on the radio. It is used a lot for 3D stuff b/c you often have a ton of control throws, but need to be able to make little precise control inputs. The part about only allowing the servo to operate within a certain range is called ATV on Futaba radios (at least on the 8U). Hope this helps!